Gas turbines are widely used in commercial operations for power generation. FIG. 1 illustrates a typical gas turbine 10 known in the art. As shown in FIG. 1, the gas turbine 10 generally includes a compressor 12 at the front, one or more combustors 14 around the middle, and a turbine 16 at the rear. The compressor 12 and the turbine 16 typically share a common rotor 18. The compressor 12 progressively compresses a working fluid and discharges the compressed working fluid to the combustors 14. The combustors 14 inject fuel into the flow of compressed working fluid and ignite the mixture to produce combustion gases having a high temperature, pressure, and velocity. The combustion gases exit the combustors 14 and flow to the turbine 16 where they expand to produce work.
FIG. 2 provides a simplified cross-section of a combustor 20 known in the art. A casing 22 surrounds the combustor 20 to contain the compressed working fluid from the compressor 12. Nozzles 24 are arranged in an end cover 26, for example, with primary nozzles 28 radially arranged around a secondary nozzle 30 as shown in FIG. 2. A liner 32 downstream of the nozzles 28, 30 defines an upstream chamber 34 and a downstream chamber 36 separated by a throat 38. The compressed working fluid from the compressor 12 flows between the casing 22 and the liner 32 to the primary 28 and secondary 30 nozzles. The primary 28 and secondary 30 nozzles mix fuel with the compressed working fluid, and the mixture flows from the primary 28 and secondary 30 nozzles into the upstream 34 and downstream 36 chambers where combustion occurs.
During full speed base load operations, the flow rate of the fuel and compressed working fluid mixture through the primary 28 and secondary 30 nozzles is sufficiently high so that combustion occurs only in the downstream chamber 36. During reduced power operations, however, the primary nozzles 28 operate in a diffusion mode in which the flow rate of the fuel and compressed working fluid mixture from the primary nozzles 28 is reduced so that combustion of the fuel and the compressed working fluid mixture from the primary nozzles 28 occurs in the upstream chamber 34.
Lower reactivity fuels, such as natural gas, typically have lower flame speeds. Due to lower natural gas flame speed, the flow rate of the fuel and compressed working mixture from the primary nozzles 28 operated in diffusion mode is sufficiently high so that combustion in the upstream chamber 34 occurs at a sufficient distance from the primary nozzles 28 to prevent the combustion from excessively heating and/or melting the primary nozzles 28. However, higher reactivity fuels, such as synthetic gas, hydrogen, carbon monoxide, ethane, butane, propane, or mixtures of higher reactivity hydrocarbons, typically have higher flame speeds. Increased flame speed of the higher reactivity fuels moves the combustion in the upstream chamber 34 closer to the primary nozzles 28. Local flame temperature under diffusion mode operation in the upstream chamber 34 can be much greater than the melting point of the primary nozzle 28 materials. As a result, primary nozzles 28 operated in diffusion mode may experience excessive heating, resulting in premature and/or catastrophic failure.
Therefore the need exists for an improved fuel flow system through the nozzles that can cool the nozzles and prevent the nozzles from melting.